class LAB6{

    public static void main(String[] args){//only for testing purposes

        int[] testArray = {2,3,4,2,5,12,2,5};
        LispList<Integer> ar = LispList.empty();
        for(int i = testArray.length-1; i>=0; i--){

            ar = ar.cons(testArray[i]);

        }

        System.out.println(positions(ar, 2));

    }

    public static LispList<Integer> positions(LispList<Integer> ar, Integer num){

        if(ar.tail().isEmpty()){// has only one element

            if(ar.head().equals(num)){
                LispList<Integer> ar2 = LispList.empty();
                return ar2.cons(0);
            }
            LispList<Integer> ar3 = LispList.empty();
            return ar3;
        }
        int counter =0;
        Integer head = ar.head();
        if(head.equals(num)){

            LispList<Integer> ar4 = positionsHelper(ar.tail(), num, counter);
            return ar4.cons(counter);

        }
        return positionsHelper(ar.tail(), num, counter);
    }

    private static LispList<Integer> positionsHelper(LispList<Integer> ar, Integer num, Integer counter){

        counter++;

        if(ar.tail().isEmpty()){

            if(num.equals(ar.head())){

                LispList<Integer> ar2 = LispList.empty();
                return ar2.cons(counter);

            }
            LispList<Integer> ar3 = LispList.empty();
            return ar3;

        }
        Integer head = ar.head();
        if(num.equals(head)){
            LispList<Integer> ar4 = positionsHelper(ar.tail(), num ,counter);
            return ar4.cons(counter);

        }
        return positionsHelper(ar.tail(), num, counter);


    }
}